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Life-history theory
Life history theory is a method of analysis in animal and human biology, psychology, and especially evolutionary sociobiology which postulates that many of the physiological traits and behaviors of individuals may be best understood in relation to the key maturational and reproductive characteristics that define the life course. Life history characteristics Life history characteristics are traits that affect the life table of an organism, and can be imagined as various investments in growth, reproduction, and survivorship. The goal of life history theory is to understand the variation in such life history strategies. This knowledge can be used to construct models to predict what kinds of traits will be favored in different environments. Without constraints, the highest fitness would belong to a Darwinian Demon, a hypothetical organism for whom such trade-offs do not exist. The key to life history theory is that there are limited resources available, and focusing on only a few life history characteristics is necessary. Examples of these characteristics include: *Age at weaning *Age of sexual maturity or puberty *Adult body size *Time to first sexual activity or mating *Time to first reproduction *Duration of gestation *Litter size *Interbirth interval Allocation of resources Variations in these characteristics reflect different allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another. For example, birds with larger broods are unable to afford more prominent secondary sexual characteristics Gustafsson, L., Qvarnström, A., and Sheldon, B.C. 1995. Trade-offs between life-history traits and a secondary sexual character in male collared flycatchers. Nature 375, 311 - 313. Life history characteristics will, in some cases, change according to the population density, since genotypes with the highest fitness at high population densities will not have the highest fitness at low population densities.Mueller, L.D., Guo, P., and Ayala, F.J. 1991. Density dependent natural selection and trade-offs in life history traits. Science, 253: 433-435. Other conditions, such as the stability of the environment, will lead to selection for certain life history traits. Experiments by Michael R. Rose and Brian Charlesworth showed that unstable environments selected for flies with both shorter lifespans and higher fecundity.Rose, M. and Charlesworth, B. A Test of Evolutionary Theories of Senescence. 1980. Nature 287, 141-142 Trade-offs Thus allocation of resources involves trade-offs. These trade-offs and strategies can be compared between species. Two of the most well-known trade-offs involve number of offspring (few or many) and timing of reproduction (accelerated maturation and reproduction versus delayed, allowing for larger size and more complex social supports). The extremes at the species level of these fundamental dimensions of reproduction were recognized long before life history theory, and are traditionally termed r/K selection theory. An r-selection strategy is the production of a large number of offspring (of whom only a minority may survive) as early in life as possible. The K-selection strategy is to produce a smaller number of "fitter" offspring with higher survival chances. According to life history theory the individuals of a species are able to make limited shifts in reproductive strategies in response to the prevailing environments. Depending on abundance of resources and probable individual longevity, individuals consciously or unconsciously shift their reproductive strategy in one direction or the other to take advantage of available resources or to compensate for resource shortage or uncertainty. Reproductive value and costs of reproduction Reproductive value models the tradeoffs between reproduction, growth, and survivorship. An organism's reproductive value (RV) is defined as its expected contribution to the population through both current and future reproductionFisher, R. A. 1930. The genetical theory of natural selection. Oxford University Press, Oxford.: RV = Current Reproduction + Residual Reproductive Value (RRV) The residual reproductive value represents an organism's future reproduction through its investment in growth and survivorship. The cost-of-reproduction hypothesis predicts that higher investment in current reproduction hinders growth and survivorship and reduces future reproduction, while investments in growth will pay off with higher fecundity (number of offspring produced) and reproductive episodes in the future. This cost-of-reproduction tradeoff influences major life history characteristics. For example, a 2009 study by J. Creighton, N. Heflin, and M. Belk on burying beetles provided "unconfounded support" for the costs of reproduction.J. Curtis Creighton, Nicholas D. Heflin, and Mark C. Belk. 2009. Cost of Reproduction, Resource Quality, and Terminal Investment in a Burying Beetle. The American Naturalist, 174:673–684. The study found that beetles that had allocated too many resources to current reproduction also had the shortest lifespans. In their lifetimes, they also had the fewest reproductive events and offspring, reflecting how over-investment in current reproduction lowers residual reproductive value. The related terminal investment hypothesis describes a shift to current reproduction with higher age. At early ages, RRV is typically high, and organisms should invest in growth to increase reproduction at a later age. As organisms age, this investment in growth gradually increases current reproduction. However, when an organism grows old and begins losing physiological function, mortality increases while fecundity decreases. This senescence shifts the reproduction tradeoff towards current reproduction: the effects of aging and higher risk of death make current reproduction more favorable. The burying beetle study also supported the terminal investment hypothesis: the authors found beetles that bred later in life also had increased brood sizes, reflecting greater investment in those reproductive events.J. Curtis Creighton, Nicholas D. Heflin, and Mark C. Belk. 2009. Cost of Reproduction, Resource Quality, and Terminal Investment in a Burying Beetle. The American Naturalist, 174:673–684. Application to humans Life histories of individual human beings may be analyzed from the same perspectives, and the theory provides some explanatory and predictive power when applied to groups of people in different life situations. For example, it predicts that in a stressful environment, or one with uncertain resources, sexual development, mating, and pregnancy may be accelerated, with acceptance of higher risks and production of more children, while security and high resource availability tend to favor slower maturation, later mating, and fewer offspring. Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility. A number of statistical predictions have been confirmed by social data, though not always reproducibly. The implications for social policy have been hotly debated because statistical associations are not always causal, and a preferred interpretation may be a more minor factor than another unpalatable relationship. See also * Behavioral ecology * Inclusive fitness * Kin selection * Parental investment * Parent-offspring conflict * Reciprocal altruism * Sexual selection * Somatic effort Also, applications to the study of human behavior: * Dual inheritance theory * Evolutionary psychology ** Evolutionary developmental psychology ** Evolutionary educational psychology * Human behavioral ecology * List of publications on evolution and human behavior References Life-history theory *Charnov, E. L. (1993). Life history invariants. Oxford, England: Oxford University Press. *Ellis, B.J. (2004). Timing of pubertal maturation in girls: an integrated life history approach. Psychological Bulletin. 130:920-58. *Roff, D. (1992). The evolution of life histories: Theory and analysis. New York:Chapman & Hall. *Roff D A (2002). Life History Evolution. Sinauer Associates Inc, MassachusettsISBN 0-87893-756-0 *Stearns, S. (1992). The evolution of life histories. Oxford, England: Oxford University Press. Further reading *Google Scholar * Borgerhoff Mulder, M. (2000). Optimizing offspring: The quality-quantity tradeoff in agropastoral Kipsigis. Evolution and Human Behavior 21(6):390-410. Full text (Click on "Publications" and then title) * Ellis, B.J. (2004). Timing of pubertal maturation in girls: An integrated life history approach. Psychological Bulletin, 130 , 920-958. Full text * Geary, D. C. (2002). Sexual selection and human life history. In R. Kail (Ed.), Advances in child development and behavior (Vol 30, pp. 41-101). San Diego, CA: Academic Press. Full text * Gurven, M., & Kaplan, H. (2006). Determinants of time allocation to production across the lifespan among the Machiguenga and Piro Indians of Peru. Human Nature 17, 1, 1-49. Full text (Click "Papers," and then click title.) * Hagen, E. H., Barrett, H. C. and Price, M. E. (in press). Do human parents face a quantity-quality tradeoff? Evidence from a Shuar community. American Journal of Physical Anthropology. Full text * Hawkes, K. & Blurton Jones, N.J. (2005). Human age structures, paleodemography, and the Grandmother Hypothesis. In E. Voland, A. Chasiotis, and W. Schiefenhovel (Eds.), Grandmotherhood: The Evolutionary Significance of the Second Half of Female Life pp. 118-140. New Brunswick: Rutgers University Press (corrected from the published version). Full text *Helle, S., Lummaa, V. & Jokela., J. (2005). Late, but not early, reproduction correlated with longevity in historical Sami women. Proceedings of the Royal Society of London: Biological Sciences, 272, 29-37. Full text * Helle, S., Lummaa, V. & Jokela J. (2004). Selection for increased brood size in pre-industrial humans. Evolution, 52, 430-436. Full text * Kaplan, H.S., & Robson, A.J. (2002). The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers. PNAS, 99, 15, 10221-10226. Full text * Kaplan, H., Hill, K., Lancaster, J., & Hurtado, A.M. (2000). A Theory of Human Life History Evolution: Diet, Intelligence, and Longevity''. Evolutionary Anthropology, 9, 4, 156-184. Full text * Mace,R., (2000). Evolutionary Ecology of Human Life History. ''Animal Behaviour. vol. 59, issue 1, 1-10.Full text * Sear, R., Mace, R. & McGregor, I.A. (2003). A life-history analysis of fertility rates in rural Gambia: evidence for trade-offs or phenotypic correlations? In J. Rodgers & H.P. Kohler (Eds.), The Biodemography of Human Reproduction and Fertility, Kluwer Press. Boston. pp 135-160. Full text * Quinlan, R., (2003). Father-Absence, Parental Care & Female Reproductive Development. Evolution and Human Behavior. 24(6): 376-390. Full text * Vigil, J. M., Geary, D. C., & Byrd-Craven, J. (2005). A life history assessment of early childhood sexual abuse in women. Developmental Psychology, 41, 553-561. Full text * Walker, R., Gurven, M., Hill, K., Migliano, A., Chagnon, N., Djurovic, G., Hames, R., Hurtado, A.M., Kaplan, H., Oliver, W., de Souza, R., Valeggia, C., Yamauchi, T. (2006). Growth rates, developmental markers and life histories in 21 small-scale societies. American Journal of Human Biology, 18, 295-311. Full text (Click "Papers," and then click title.) Category:Biology Category:Evolutionary developmental psychology Category:Human behavioral ecology Category:Human development Category:Psychological theories